Fwd: C-[ is undefined

[Michael Brand <michael.ch.brand@gmail.com>]

There seems to be no answer yet but I am still stuck and would
appreciate any help to get further.

---------- Forwarded message ----------
From: Michael Brand <michael.ch.brand@gmail.com>
Date: Sat, Jul 7, 2018 at 1:52 PM
Subject: C-[ is undefined
To: help-gnu-emacs@gnu.org

Hi all

Short story: ~C-x @ c [~ results in "C-[ is undefined" but I expected
it to be the Meta prefix like in for example ~C-[ t~ for ~M-t~.

Long story: With the help of key-chord-mode I started to use two
simultaneously pressed normal keys to get the control modifier:

#+begin_src emacs-lisp
  (defvar v-key-chord-control "en"
    "Key chord for `event-apply-control-modifier'.
  The key chord is optimized for `vi-state' and the Colemak
  keyboard layout (where `en' is QWERTY `jk').")

  (key-chord-define key-translation-map v-key-chord-control
                    #'event-apply-control-modifier)
  (key-chord-define-global v-key-chord-control
                           #'event-apply-control-modifier)
#+end_src

Now also ~en [~ results in "C-[ is undefined". The same applies for \,
], ^ and _ with control modifier. All other control characters (means
a-z with control modifier like for example "en s" for I-search) are
bound as expected.

I tried

#+begin_src emacs-lisp
  (define-key key-translation-map (kbd "C-[") [(escape)])
#+end_src

or

#+begin_src emacs-lisp
  (global-set-key (kbd "C-[") [(escape)])
#+end_src

but ~C-[~ remains undefined. What is the part that I am missing? (And
what will be needed instead of ~[(escape)]~ after the binding is
fixed?)

Please CC me as I am not subscribed to this list.

Michael